Fatigue failure is one of the most important failure modes of engineering structures, which threatens the safety and causes huge economic losses. Therefore, various countries in the world attach great importance to researches in the field of fatigue fracture. A lot of scholars at home and abroad have made a great deal of researches on the theory of uniaxial fatigue, and accumulated rich experience and valuable data. However, there are few researches on the multiaxial fatigue of building structures in China. Some special or new structural details in the steel bridge structures, such as a cable-girder anchorage zone of a cable-stayed bridge, an orthotropic steel bridge deck, an integral joint and a pipe structure welding node and the like, show stronger multi-axis effect in fatigue behavior. Therefore, the research of multiaxial fatigue is closer to engineering practice than uniaxial fatigue.
For the research of structure fatigue under a constant amplitude loading action, an S-N curve of the material can be used to estimate the number of cycles experienced to damage at different stress levels. However, for cyclic loading at two or more stress levels, it is not possible to estimate the service life directly using the S-N curve, and the fatigue cumulative damage criterion needs to be used as well. The researches on cumulative fatigue damage have been lasted for decades, but a satisfactory uniform model has not been made so far. Moreover, the actual structure is often in a complex state of stress, so that a lot of experiments are also required to research the structure fatigue damage rules.
At present, the research on structure fatigue tests is mainly based on uniaxial fatigue tests; while the multiaxial tests still depend on large-scale loading equipment such as MTS and the like, so that the test has low efficiency and is uneconomical. Through searching, China patent document CN103994936A discloses a horizontal high-tonnage energy-saving fatigue experiment device which is applied to the field of mechanical design and metal material performance testing. The horizontal high-tonnage energy-saving fatigue experiment device includes a shell, an actuating cylinder, an actuating cylinder piston rod, actuating cylinder oil inlet and outlet pipelines, hydraulic servo flow valves, a first energy accumulating device, a second energy accumulating device and an electromagnet loading device; wherein the electromagnet loading device comprises a soft magnet, an electric magnet, a load sensor, an electromagnet loading actuating rod and an electromagnet controller. The electromagnet loading device is combined with a traditional dual-output-rod high-tonnage fatigue testing machine. The electromagnet loading starts until the existing high-tonnage actuating cylinder is loaded to an average load as required for test, for saving the energy sources as required for the actuating cylinder to keep pressure, the actuating cylinder is also required while starting, and limits to the uniaxial fatigue test.
Based on the above circumstances, designing a novel economic multiaxial fatigue testing machine capable of simulating actual structure loading becomes an urgent technical problem to be solved.